The invention relates to an optical recording medium comprising a cross-linked buffer layer, a method for the manufacture thereof and an apparatus for the continuous manufacture of the same.
More particularly the invention relates to optical recording media such as compact discs (CDs) and digital tapes or cards, usually known as WORM media (write-once-read-many-times) and rewritable CDs and tapes. These media allow digital information such as data, music, and image, to be written by the user.
In conventional read-only CDs the information is stored in pits embossed in the disc. The reading is based on diffraction of reflected light on the regular pit structure. Interference of the diffracted orders is dependent on the position of the laser spot. This results in a modulation in reflection, which is used for reading the information. The conventional read-only CDs are only suitable for large-scale production as the production steps (for obtaining a written disc) are rather complicated and therefore only cost-effective in mass production. Hence there is need for CDs and digital tapes or cards which can be produced in smaller quantities or can even be written by the user himself. In EP-A2-0353391 an optical recording medium is described comprising a light-transmitting substrate having a deformable surface, a light-absorptive layer overlaying the deformable surface, and a light-reflective layer overlaying the light-absorptive layer, said deformable surface being deformable by energy generated upon absorption of the writing laser beam by the light-absorptive layer, to form optically readable pits. The reading is again based on pit-edge interference. During irradiation with the reading laser the light travels through the light-absorptive layer and is reflected by the reflective layer. As the refractive index within a pit differs from the refractive index outside it (land), the optical path length within the pit differs from that of the land. The laser light which falls within the pit interferes with the light which falls on the land. The resulting reflection modulation at this pit/land edge is used for reading the information. There is still need for improvement in reflectivity and contrast in the WORM media and rewritable media proposed so far. In our co-pending PCT patent application WO 96/16402 an optical recording medium is described which comprises a substrate provided with a partially transparent thin reflective layer, which is provided with a layer comprising liquid-crystalline material having a thickness between 100 and 1200 nm, which is provided with a thick layer having a reflectance over 50%. Thus, in comparison with the above-described optical recording media, an extra thin reflective layer is present, resulting in the liquid-crystalline material layer being sandwiched between two reflective layers. In this way a Fabry-Perot interferometer is created. The Fabry-Perot phenomenon is used to obtain a difference in reflection between the written and the unwritten state in the digital storage medium. Although a satisfactory contrast can be obtained with the concept described in application WO 96/16402, several problems still have to be solved. Some of these problems have been solved in co-pending, not yet published European patent application 95203502.0. It has now been found that substantial further improvements, in particular an improved writing contrast and an improved manufacturing process, can be obtained by applying a transparent buffer layer comprising a material which is capable of cross-linking, and an initiator which is preferably a photo-initiator.